This invention relates to digital communications. More specifically, the present invention relates to receiver circuits which compensate for system-induced distortions.
Conventional digital communication systems convey digital data in accordance with a transmitter-implemented phase point constellation. This constellation defines a finite number of phase states to be used in conveying data to a receiver. FIG. 1 shows an exemplary prior art transmitter-implemented constellation 10 configured in accordance with the well-known 16-QAM modulation technique. Sixteen phase points 12 reside in a phase space 14 at all combinations of xe2x88x923, xe2x88x921, +1, and +3 values of orthogonal in-phase (I) and quadrature (Q) components of a communication signal. In 16-QAM, phase points 12 reside at one of a greater, intermediate, or lesser magnitude away from an origin 16 of phase space 14, as indicated by magnitude vectors 18, 20, and 22, respectively. Moreover, in 16-QAM each phase point 12 has its own unique four-bit code. In the example depicted in FIG. 1, two bits to the right of a radix point in this four-bit code are encoded for the purposes of error correction, such as may occur through a convolutional encoder. During each unit baud interval, the communication signal conveys a single four-bit code.
A receiver then processes the communication signal to detect the four-bit code conveyed during each unit baud interval. Generally, the receiver detects transmitted data by comparing a phase estimate for a unit baud interval with data compiled in response to constellation 10. However, noise and other influences corrupt the received communication signal so that a received estimate may reside at any location in phase space 14 rather than the location from which transmission occurred.
The error correction encoding (e.g., the two least significant bits of the four-bit codes depicted in FIG. 1) is helpful in resolving the transmitted data. Error correction involves the translation of phase estimates in the receiver into log-likelihood ratios in a branch metrics generator, which are then fed to a decoder. Accordingly, the branch metrics generator transfer function plays an important role in correctly deciphering encoded data.
FIG. 1 illustrates an exemplary conventional branch metrics transfer function for the 16-QAM example. In particular, FIG. 1 graphically illustrates transfer functions for four segments of a branch metric generator at the Q=xe2x88x923 contour of phase space 14. The four segments refer to respective branch metrics outputs (L00, L01, L11, and L10) which characterize the likelihoods that the two encoded bits convey values of 00, 01, 11, or 10 over each fundamental unit area of the receiver phase space. As illustrated in FIG. 1, the branch metrics transfer function experiences peaks and valleys at the same symbol group amplitude levels, e.g. xe2x88x923, xe2x88x921, +1, and +3 used in transmitter-implemented constellation 10. Consequently, the ratio of a greater magnitude peak 24 to a lesser magnitude peak 26 in the received phase space characterized in the branch metrics generator equals the ratio of maximum magnitude vector 18 to minimum magnitude vector 22 in transmitter-implemented constellation 10. While FIG. 1 illustrates only the well-known 16-QAM modulation technique, this conventional technique is universally applied, even with transmitter-implemented phase constellations that distribute outer phase points further apart from one another than inner phase points.
The use of equal symbol group amplitude levels in transmitter and receiver phase spaces, and the resulting equal ratios of greater to lesser magnitude phase points in transmitter and receiver, result at least in part from a random additive and multiplicative noise assumption employed in conventional digital communication receiver design. In short, conventional receivers and branch metric generators therein are designed using the assumption that a displacement of a received phase estimate from its ideal phase point is as likely to occur in one direction in phase space as in another, and that displacements of equal magnitude are equally likely for any phase point in the constellation, or at least for phase points having equal magnitude.
While the random noise assumption has produced acceptable results in prior digital communication receivers, digital communication performance requirements are evolving so that a need now exists to depart from this random noise assumption to achieve improved performance.
Accordingly, it is an advantage of the present invention that an improved distortion-compensated digital communications receiver and method are provided.
Another advantage is that a digital communication receiver is configured to accommodate non-random components of phase error as well as random components of phase error.
Another advantage is that a digital communication receiver has a branch metrics generator configured to accommodate non-random components of phase error as well as random components of phase error.
Another advantage is that a method is provided for calculating branch metrics to accommodate non-random components of phase error.
The above and other advantages of the present invention are carried out in one form by a distortion-compensated receiver for digital communication configured in accordance with a transmitter-implemented constellation having greater magnitude and lesser magnitude phase points. The ratio of a magnitude for the greater magnitude phase point divided by a magnitude for the lesser magnitude phase point is RTX. The distortion-compensated receiver includes a phase estimator configured to supply phase estimate values and a branch metrics generator coupled to the phase estimator. The branch metrics generator has a generator segment for a phase space exhibiting a transfer function with first and second peaks respectively corresponding to the greater and lesser magnitude phase points, where the ratio of a phase space magnitude for the first likelihood peak divided by a phase space magnitude for the second likelihood peak is RRX, and where RRX is less than RTX. A decoder couples to the branch metrics generator.
The above and other advantages of the present invention are carried out in another form by a method for generating branch metrics. The method calls for generating phase estimates within a quantized phase space for a received data communication signal. In response to this generating activity, probabilities for detecting a phase estimate at each quantum in said quantized phase space are empirically determined. The probabilities form a probability density matrix. Log-likelihood values are calculated in response to the probability density matrix.